This invention relates to a Stirling engine and particularly to a system for producing a working fluid for the Stirling engine.
The Stirling engine is based on a thermodynamic principle similar to that of an internal combustion engine, namely, if gas is compressed at low temperature and then is heated and allowed to expand, mechanical energy is produced. In the Stirling engine, however, the method of heating the gases is different from that of the internal combustion engine.
In that Stirling engines are operated on a closed thermodynamic cycle, they are reversible in terms of their thermodynamic output. In one configuration, heat is absorbed by the engine and converted to mechanical work through a rotating shaft. In another operating condition, mechanical power input can be converted to thermal outputs in the terms of cooling or heating capacity.
The working gas or working fluid, which in modern Stirling engines is typically helium or hydrogen, is shuttled from a space where it is at a constant high temperature into a space at which it is a constant low temperature. In order to obtain mechanical energy from this process, the working gas must be compressed where it is mainly in the cold space and allowed to expand where it is mainly in the hot space. In order not to lose heat during this shuttling process, a regenerator is placed between the hot and cold spaces. A regenerator is a space filled with course material, such as layers of very fine metallic gauze. The material captures the heat of the gas as it flows from the hot space to the cold space, and returns its heat to the gas on its way back to the hot space.
The assignee of the present application, STM Power, Inc., is a pioneer in the development of modern Stirling cycle engines. Its current design of its model xe2x80x9c4-120xe2x80x9d engine is a four cylinder, double-acting type using a swashplate kinematic drive. Patents describing this basic engine configuration include U.S. Pat. Nos. 4,996,841; 5,074,114; 5,611,201; 5,706,659; 5,722,239; 5,771,694; 5,813,229; 5,836,846; and 5,864,770 which are hereby incorporated by reference.
Although presently available Stirling engines have enormous potential for commercial applications, there are certain design challenges which remain. Presently, hydrogen is the preferred working gas for Stirling engines, since it provides higher overall thermal conversion efficiency than provided by the use of helium as a working gas. The use of hydrogen has a number of drawbacks however. Hydrogen, being the simplest element, has the smallest atomic size known and therefore escapes through various leakage paths or by diffusion through solid materials.
In the Stirling engine configurations produced by the Assignee, hydrogen losses may occur at various locations. The kinematic drive system for the displacer pistons is coupled to the pistons through the use of reciprocating shafts. The reciprocating shafts pass through sliding contact seals which are provided to isolate the hydrogen working fluid from atmosphere. The requirements for sealing hydrogen in the environment of a reciprocating sliding contact is a significant design challenge. Highly sophisticated and costly sealing systems may be used for this application to reduce or virtually eliminate hydrogen leakage. For example, bellows-types and other sealing arrangements have been considered. Although sliding contact rod seals and bellow-type arrangements are capable of significantly reducing the loss of hydrogen gas, they are often costly to produce, sensitive to wear, and in some cases, my not have sufficient durability lifetimes for the intended applications.
Another principal point of loss of hydrogen gas occurs at the heater head of the engine. Since it is necessary to heat the gas at one side of its cycle, a heater head is provided which is exposed to hot combustion gases, solar energy, or other heat sources. The elevated temperature of the materials which comprise the heater head of the engine further exacerbates diffusion losses. Diffusion is a transport mechanism in which hydrogen travels directly through microscopic voids in the material and this process is accelerated at high temperatures. Numerous technologies related to the use of coatings and other approaches to reducing this source of gas loss, have also been considered. Once again, costs and other factors are drawbacks. Hydrogen gas loss is often divided into static leakage occurring when the machine is not operating, and dynamic leakage during operation.
There are numerous potential applications for Stirling engines which require long term efficient operation. For those applications, it is necessary to ensure that the sufficient charge of working gas remains within the engine through the operational lifetime. As mentioned previously, one approach is to minimize all losses of working fluid. If such losses can be maintained at a sufficiently low rate, it may be possible to periodically supplement the working gas by xe2x80x9crechargingxe2x80x9d the engine as it is serviced. This solution may, however, not be sufficient in certain applications unless leakage rate and service intervals are appropriate.
This invention addresses the problem of Stirling engine working gas loss through another approach; namely, to produce in a subsystem coupled directly with the Stirling engine sufficient hydrogen working gas to supplement the initial charge upon the occurrence of loss of hydrogen working gas. If sufficient quantities of hydrogen gas may efficiently produced such a mechanism, a certain rate of hydrogen loss may be tolerated. Through this approach, substantial extensions of operating lifetime are achievable. Since the system is preferably portable with the engine, it is available to supplement the working gas charge wherever the engine travels, which is especially important for motor vehicle, portable or airborne systems implementing Stirling engines, or otherwise where access is inconvenient or unavailable.
In accordance with this invention, an onboard hydrogen gas production system is provided which incorporates a reservoir of a hydrogen containing material such as ordinary water. This liquid is reformed by electrolysis, thus separating it into its fundamental elements which, in the case of water, are oxygen and hydrogen. The hydrogen produced in this manner is stored in a reservoir and preferably a reservoir containing a metal hydride storage material. By controlling the temperature of the metal hydride in the reservoir, hydrogen which has been stored can be liberated and pumped into the Stirling engine as needed. This system responds to a charge state signal from the Stirling engine related to the state of working gas charge.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims taken in conjunction with the accompanying drawings.